Timing control for two-chamber gas discharge laser system

ABSTRACT

Feedback timing control equipment and process for an injection seeded modular gas discharge laser. A preferred embodiment is a system capable of producing high quality pulsed laser beams at pulse rates of about 4,000 Hz or greater and at pulse energies of about 5 to 10 mJ or greater for integrated outputs of about 20 to 40 Watts or greater. The feedback timing control is programmed to permit in some circumstances discharges timed so that no significant laser energy is output from the system. Use of this technique permits burst mode operation in which the first discharge of a burst is a no-output discharge so that timing parameters for each of the two chambers can be monitored before the first laser output pulse of the burst. Two separate discharge chambers are provided, one of which is a part of a master oscillator producing a very narrow band seed beam which is amplified in the second discharge chamber. The chambers can be controlled separately permitting optimization of wavelength parameters in the master oscillator and optimization of pulse energy parameters in the amplifying chamber.

[0001] The present invention is a continuation-in-part of Ser. No.______, filed Nov. 30, 2001, Ser. No. 09/943,343, filed Aug. 29, 2001and Ser. No. 09/848,043, filed May 3, 2001, each of which areincorporated by reference herein. This invention relates to narrow bandtwo chamber gas discharge laser systems and in particular to dischargetiming controls for such systems.

BACKGROUND OF THE INVENTION Electric Discharge Gas Lasers

[0002] Electric discharge gas lasers are well known and have beenavailable since soon after lasers were invented in the 1960s. A highvoltage discharge between two electrodes excites a laser gas to producea gaseous gain medium. A resonance cavity containing the gain mediumpermits stimulated amplification of light which is then extracted fromthe cavity in the form of a laser beam. Many of these electric dischargegas lasers are operated in a pulse mode.

Excimer Lasers

[0003] Excimer lasers are a particular type of electric discharge gaslaser and they have been known since the mid 1970s. A description of anexcimer laser, useful for integrated circuit lithography, is describedin U.S. Pat. No. 5,023,884 issued Jun. 11, 1991 entitled “CompactExcimer Laser.” This patent has been assigned to Applicants' employer,and the patent is hereby incorporated herein by reference. The excimerlaser described in patent '884 is a high repetition rate pulse laser.These excimer lasers, when used for integrated circuit lithography, aretypically operated in an integrated circuit fabrication line“around-the-clock” producing many thousands of valuable integratedcircuits per hour; therefore, down-time can be very expensive. For thisreason most of the components are organized into modules which can bereplaced within a few minutes. Excimer lasers used for lithographytypically must have its output beam reduced in bandwidth to a fractionof a picometer. This “line-narrowing” is typically accomplished in aline narrowing module (called a “line narrowing package” or “LNP”) whichforms the back of the laser's resonant cavity. This LNP is comprised ofdelicate optical elements including prisms, mirrors and a grating.Electric discharge gas lasers of the type described in patent '884utilize an electric pulse power system to produce the electricaldischarges, between the two electrodes. In such prior art systems, adirect current power supply charges a capacitor bank called “thecharging capacitor” or “C₀” to a predetermined and controlled voltagecalled the “charging voltage” for each pulse. The magnitude of thischarging voltage may be in the range of about 500 to 1000 volts in theseprior art units. After C₀ has been charged to the predetermined voltage,a solid state switch is closed allowing the electrical energy stored onC₀ to ring very quickly through a series of magnetic compressioncircuits and a voltage transformer to produce high voltage electricalpotential in the range of about 16,000 volts (or greater) across theelectrodes which produce the discharges which lasts about 20 to 50 ns.

Major Advances in Lithography Light Sources

[0004] Excimer lasers such as described in the '884 patent have duringthe period 1989 to 2001 become the primary light source for integratedcircuit lithography. More than 1000 of these lasers are currently in usein the most modern integrated circuit fabrication plants. Almost all ofthese lasers have the basic design features described in the '884patent.

[0005] This is:

[0006] (1) a single, pulse power system for providing electrical pulsesacross the electrodes at pulse rates of about 100 to 2500 pulses persecond;

[0007] (2) a single resonant cavity comprised of a partially reflectingmirror-type output coupler and a line narrowing unit consisting of aprism beam expander, a tuning mirror and a grating;

[0008] (3) a single discharge chamber containing a laser gas (either KrFor ArF), two elongated electrodes and a tangential fan for circulatingthe laser gas between the two electrodes fast enough to clear thedischarge region between pulses, and

[0009] (4) a beam monitor for monitoring pulse energy, wavelength andbandwidth of output pulses with a feedback control system forcontrolling pulse energy, energy dose and wavelength on a pulse-to-pulsebasis.

[0010] During the 1989-2001 period, output power of these lasers hasincreased gradually and beam quality specifications for pulse energystability, wavelength stability and bandwidth have also becomeincreasingly tighter. Operating parameters for a popular lithographylaser model used widely in integrated circuit fabrication include pulseenergy at 8 mJ, pulse rate at 2,500 pulses per second (providing anaverage beam power of up to about 20 watts), bandwidth at about 0.5 pm(FWHM) and pulse energy stability at +/−0.35%.

[0011] There is a need for further improvements in these beamparameters. Integrated circuit fabricators desire better control overwavelength, bandwidth, higher beam power with more precise control overpulse energy. Some improvements can be provided with the basic design asdescribed in the '884 patent; however, major improvements with thatbasic design may not be feasible. For example, with a single dischargechamber precise control of pulse energy may adversely affect wavelengthand/or bandwidth and vice versa especially at very high pulse repetitionrates.

Injection Seeding

[0012] A well-known technique for reducing the band-width of gasdischarge laser systems (including excimer laser systems) involves theinjection of a narrow band “seed” beam into a gain medium. In one suchsystem, a laser producing the seed beam called a “master oscillator” isdesigned to provide a very narrow bandwidth beam in a first gain medium,and that beam is used as a seed beam in a second gain medium. If thesecond gain medium functions as a power amplifier, the system isreferred to as a master oscillator, power amplifier (MOPA) system. Ifthe second gain medium itself has a resonance cavity (in which laseroscillations take place), the system is referred to as an injectionseeded oscillator (ISO) system or a master oscillator, power oscillator(MOPO) system in which case the seed laser is called the masteroscillator and the downstream system is called the power oscillator.Laser systems comprised of two separate systems tend to be substantiallymore expensive, larger and more complicated than comparable singlechamber laser systems. Therefore, commercial application of these twochamber laser systems has been limited.

Jitter Problems

[0013] In gas discharge lasers of the type referred to above, theduration of the electric discharge is very short duration, typicallyabout 20 to 50 ns (20 to 50 billions of a second). Furthermore, thepopulation inversion created by the discharge is very rapidly depletedso that the population inversion effectively exists only during thedischarge. In these two laser systems, the population in the downstreamlaser must be inverted when the beam from the upstream laser reaches thesecond laser. Therefore, the discharges of the two lasers must beappropriately synchronized for proper operation of the laser system.This can be a problem because within typical pulse power systems thereare several potential causes of variation in the timing of thedischarges. Two of the most important sources of timing variations arevoltage variations and temperature variations in saturable inductorsused in the pulse power circuits. It is known to monitor the pulse powercharging voltage and inductor temperatures and to utilize the data fromthe measurements and a delay circuit to normalize timing of thedischarge to desired values. One prior art example is described in U.S.Pat. No. 6,016,325 which is incorporated herein by reference. Thus inthe prior art, timing errors can be reduced but they could not beeliminated. These errors that ultimately result are referred to as“jitter”.

[0014] When a two chamber laser system is operating continuously thejitter problem can be dealt with by measuring the time between triggerand light out for each chamber and by providing feedback signals forsubsequent pulses based on measured timing values for previous pulsessuch as the immediately preceding pulse. This technique does not workwell; however, for the first pulse following an idle period because thetemperature of electrical components tend to drift during idle periodschanging the timing characteristics of the components.

[0015] What is needed is a better method of dealing with the jitterproblem.

SUMMARY OF THE INVENTION

[0016] The present invention provides feedback timing control equipmentand process for an injection seeded modular gas discharge laser. Apreferred embodiment is a system capable of producing high qualitypulsed laser beams at pulse rates of about 4,000 Hz or greater and atpulse energies of about 5 to 10 mJ or greater for integrated outputs ofabout 20 to 40 Watts or greater. The feedback timing control isprogrammed to permit in some circumstances discharges timed so that nosignificant laser energy is output from the system. Use of thistechnique permits burst mode operation in which the first discharge of aburst is a no-output discharge so that timing parameters for each of thetwo chambers can be monitored before the first laser output pulse of theburst. Two separate discharge chambers are provided, one of which is apart of a master oscillator producing a very narrow band seed beam whichis amplified in the second discharge chamber. The chambers can becontrolled separately permitting optimization of wavelength parametersin the master oscillator and optimization of pulse energy parameters inthe amplifying chamber. A preferred embodiment is an ArF excimer lasersystem configured as a MOPA and specifically designed for use as a lightsource for integrated circuit lithography. In this preferred embodiment,both of the chambers and the laser optics are mounted on a verticaloptical table within a laser enclosure. In the preferred MOPAembodiment, each chamber comprises a single tangential fan providingsufficient gas flow to permit operation at pulse rates of 4000 Hz orgreater by clearing debris from the discharge region in less time thanthe approximately 0.25 milliseconds between pulses. The masteroscillator is equipped with a line narrowing package having a very fasttuning mirror capable of controlling centerline wavelength on apulse-to-pulse basis at repetition rates of 4000 Hz or greater andproviding a bandwidth of less than 0.2 pm (FWHM). This preferredembodiment also includes a pulse multiplying module dividing each pulsefrom the power amplifier into either two or four pulses in order toreduce substantially deterioration rates of lithography optics. Otherpreferred embodiments are configured as KrF or F₂ MOPA laser systems.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIGS. 1 and 1A show a preferred embodiment of the presentinvention.

[0018]FIGS. 2 and 2A show chamber features.

[0019]FIGS. 3A, 3B, 3C1, 3C2, 3C3 and 3D show additional pulse powerfeatures.

[0020]FIGS. 4, 4A, 4B and 4C show features of a preferred pulse powersystem.

[0021]FIG. 5 shows laser output energy as a function of discharge timingof a MO and a PA.

[0022]FIGS. 6, 6A, 7 and 7A are flow designs showing processes forproducing a no-output first pulse.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0023] Preferred embodiments of the present invention can be describedby reference to the drawings.

Preferred Layout

[0024]FIG. 1 is a preferred general layout of a two-chamber ArFdischarge laser system configured as a master oscillator power amplifier(MOPA) system. The system includes the following features which aredescribed in much more detail in U.S. patent application Ser. No. ______filed Nov. 30, 2001. Features included are:

[0025] (1) The two chambers and the laser optics are mounted on avertical optical table 11 which is kinematically mounted within thelaser cabinet 4. The chambers are supported on stiff cantilever armsbolted to the optical table. In this design the master oscillator 10 ismounted above the power amplifier 12.

[0026] (2) A high voltage power supply 6B is contained within lasercabinet 4. This two chamber-ArF 4000 Hz system needs only a single 1200volt power supply. The same is true for a 4000 Hz KrF system. The lasercabinet, however, is provided with space for two additional high voltagepower supplies which will be needed for a two chamber, 6000 Hz, F₂ lasersystem. One additional HVPS will be utilized for a 6000 Hz ArF system.

[0027] (3) Each of the two laser chambers and the pulse power suppliesfor the chambers are substantially identical to the chamber and pulsepower supply utilized in a 4000 Hz single chamber laser system describedU.S. patent application Ser. No. 09/854,097 which has been incorporatedherein by reference.

[0028] (4) A pulse multiplier module 13 located behind the optical table11 is included in this embodiment to stretch the duration of the pulseexiting the power amplifier.

[0029] (5) The master oscillator beam output optics 14A directs theoutput beam from the MO to the power amplifier input-output optics 14Band for two passes through the power amplifier 12 via power amplifierrear optics 14C. The first pass is at a small angle with the electrodesand the second pass is aligned with the electrodes. The entire beam paththrough the laser system including the pulse stretcher is enclosed invacuum compatible enclosures (not shown) and the enclosures are purgedwith nitrogen or helium.

The Master Oscillator

[0030] The master oscillator 10 shown in FIG. 1 is in many ways similarto prior art ArF lasers such as described in the '884 patent and in U.S.Pat. No. 6,128,323 and is substantially equivalent to the ArF laserdescribed in U.S. patent application Ser. No. 09/854,097 except theoutput pulse energy is about 0.1 mJ instead of about 5 mJ. Majorimprovements over the '323 laser are provided to permit operation at4000 Hz and greater. The master oscillator is optimized for spectralperformance including bandwidth control. This result is a much morenarrow bandwidth and improved bandwidth stability. The master oscillatorcomprises discharge chamber as shown in FIG. 2 and FIG. 2A in which arelocated a pair of elongated electrodes 10A-2 and 10A-4, each about 50 cmlong and spaced apart by about 0.5 inch. Anode 10A-4 is mounted on flowshaping anode support bar 10A-6. Four separate finned water cooled heatexchanger units 10A-8 are provided. A tangential fan 10A-10 is driven bytwo motors (not shown) for providing a laser gas flow at a velocity ofabout 80 m/s between the electrodes. The chamber includes window units(not shown) with CaF₂ windows positioned at about 45° with the laserbeam. An electrostatic filter unit having an intake at the center of thechamber, filters a small portion of the gas flow as indicated at 11 inFIG. 2 and the cleaned gas is directed into window units in the mannerdescribed in U.S. Pat. No. 5,359,620 (incorporated herein by reference)to keep discharge debris away from the windows. The gain region of themaster oscillator is created by discharges between the electrodesthrough the laser gas which in this embodiment is comprised of about 3%argon, 0.1% F₂ and the rest neon. The gas flow clears the debris of eachdischarge from the discharge region prior to the next pulse. Theresonant cavity is created at the output side by an output coupler whichis comprised of a CaF₂ mirror mounted perpendicular to the beamdirection and coated to reflect about 30% of light at 193 nm and to passabout 70% of the 193 nm light. The opposite boundary of the resonantcavity is a line narrowing unit 10C as shown in FIG. 1 similar to priorart line narrowing units described in U.S. Pat. No. 6,128,323. The LNPis described in more detail below as in FIGS. 16, 16A, 16B1 and 16B2.Important improvements in this line narrowing package include four CaFbeam expanding prisms for expanding the beam in the horizontal directionby 45 times and a tuning mirror controlled by a stepper motor forrelatively large pivots and a piezoelectric driver for providingextremely fine tuning of the mirror. Echelle grating 10C3 having about80 facets per mm is mounted in the Litrow configuration reflects a verynarrow band of UV light selected from the approximately 300 pm wide ArFnatural spectrum. Preferably the master oscillator is operated at a muchlower F2 concentration than is typicaly used in prior art lithographylight sources. This results in substantial reductions in the bandwidth.Another important improvement is a narrow rear aperture which limits thecross section of the oscillator beam to 1.1 mm in the horizontaldirection and 7 mm in the vertical direction.

[0031] In preferred embodiments the main charging capacitor banks forboth the master oscillator and the power amplifier are charged inparallel so as to reduce jitter problems. This is desirable because thetime for pulse compression in the pulse compression circuits of the twopulse power systems is dependent on the level of the charge of thecharging capacitors. Preferably pulse energy output is controlled on apulse-to-pulse basis by adjustment of the charging voltage. This limitssomewhat the use of voltage to control beam parameters of the masteroscillator. However, laser gas pressure and F₂ concentration can beeasily controlled to achieve desirable beam parameters over a wide rangepulse energy increases and laser gas pressure. Bandwidth decreases withF₂ concentration and laser gas pressure. These control features are inaddition to the LNP controls. (For the master oscillator the timebetween discharge and light-out is a function of F₂ concentration (1ns/kPa), so F₂ concentration may be changed to vary the timing.)

Power Amplifier

[0032] The power amplifier in preferred embodiments is comprised of alaser chamber which is very similar to the corresponding masteroscillator discharge chamber. Having the two separate chambers allowsthe pulse energy and integrated energy in a series of pulses (calleddose) to be controlled, to a large extent, separately from wavelengthand bandwidth. This permits better dose stability. All of the componentsof the chamber are the same and are interchangeable during themanufacturing process. However, in operation, the gas pressure ispreferably substantially lower in the MO as compared to the PA. Thecompression head of the power amplifier is also substantially identicalin this embodiment to the compression head and the components of thecompression head are also interchangeable during manufacture. Onedifference is that the capacitors of the compression head capacitor bankare more widely positioned for the MO to produce a substantially higherinductance as compared to the PA. This close identity of the chambersand the electrical components of the pulse power systems helps assurethat the timing characteristics of the pulse forming circuits are thesame or substantially the same so that jitter problems are minimized.

[0033] The power amplifier is configured for two beam passages throughthe discharge region of the power amplifier discharge chamber. Thecharging voltages preferably are selected on a pulse-to-pulse basis tomaintain desired pulse and dose energies. F₂ concentration and laser gaspressure can be adjusted to provide a desired operating range ofcharging voltage. This desired range can be selected to produce adesired value of dE/dV since the change in energy with voltage is afunction of F₂ concentration and laser gas pressure. The timing ofinjections is preferable based on charging voltage. The frequency ofinjections is preferably high to keep conditions relatively constant andcan be continuous or nearly continuous. Some users of these embodimentsmay prefer larger durations (such as 2 hours) between F₂ injections.

Pulse Power Circuit

[0034] In the preferred embodiment shown in FIG. 1, the basic pulsepower circuits are similar to pulse power circuits of prior art excimerlaser light sources for lithography. However, separate pulse powercircuits downstream of the charging capacitors are provided for eachdischarge chamber. Preferably a single resonant charger charges twocharging capacitor banks connected in parallel to assure that bothcharging capacitor banks are charged to precisely the same voltage.Important improvements are also provided to regulate the temperature ofcomponents of the pulse power circuits. In preferred embodiments thetemperatures of the magnetic cores of saturable inductors are monitoredand the temperature signals are utilized in a feedback circuit to adjustthe relative timing of the discharge in the two chambers. FIGS. 3A and3B show important elements of a preferred basic pulse power circuitwhich is used for the MO. The same basic circuit is also used for thePA.

Resonant Charger

[0035] A preferred resonant charger system is shown in FIG. 3B. Theprincipal circuit elements are:

[0036] I1—A three-phase power supply 300 with a constant DC currentoutput.

[0037] C-1—A source capacitor 302 that is an order of magnitude or morelarger than the C₀ capacitor 42 shown in FIG. 3A.

[0038] Q1, Q2, and Q3—Switches to control current flow for charging andmaintaining a regulated voltage on C₀.

[0039] D1, D2, and D3—Provides current single direction flow.

[0040] R1, and R2—Provides voltage feedback to the control circuitry.

[0041] R3—Allows for rapid discharge of the voltage on C₀ in the eventof a small over charge.

[0042] L1—Resonant inductor between C-1 capacitor 302 and Co capacitorbanks 42 to limit current flow and setup charge transfer timing.

[0043] Control Board 304—Commands Q1, Q2, and Q3 open and closed basedupon circuit feedback parameters.

[0044] This circuit includes switch Q2 and diode D3, together known as aDe-Qing switch. This switch improves the regulation of the circuit byallowing the control unit to short out the inductor during the resonantcharging process. This “de-qing” prevents additional energy stored inthe current of the charging inductor, L1, from being transferred tocapacitor C₀.

[0045] Prior to the need for a laser pulse, the voltage on C-1 ischarged to about 1500 volts and switches Q1-Q3 are open. Upon commandfrom the laser, Q1 would close. At this time current would flow from C-1to C₀ through the charge inductor L1. As described in the previoussection, a calculator on the control board would evaluate the voltage onC₀ and the current flowing in L1 relative to a command voltage set pointfrom the laser. Q1 will open when the voltage on the CO capacitor banksplus the equivalent energy stored in inductor L1 equals the desiredcommand voltage. The calculation is:

V _(f) =[V _(C0s) ²+((L ₁ *I _(L1s) ²)/C ₀)]^(0.5)

[0046] Where:

[0047] V_(f)=The voltage on C₀ after Q1 opens and the current in L1 goesto zero.

[0048] V_(c0s)=The voltage on C₀ when Q1 opens.

[0049] I_(L1s)=The current flowing through L₁ when Q1 opens.

[0050] After Q1 opens the energy stored in L1 starts transferring to theCO capacitor banks through D2 until the voltage on the CO capacitorbanks approximately equals the command voltage. At this time Q2 closesand current stops flowing to CO and is directed through D3. In additionto the “de-qing” circuit, Q3 and R3 from a bleed-down circuit allowadditional fine regulation of the voltage on CO.

[0051] Switch Q3 of bleed down circuit 216 will be commanded closed bythe control board when current flowing through inductor L1 stops and thevoltage on C₀ will be bled down to the desired control voltage; thenswitch Q3 is opened. The time constant of capacitor C_(o) and resistorR3 should be sufficiently fast to bleed down capacitor C_(o) to thecommand voltage without being an appreciable amount of the total chargecycle.

[0052] As a result, the resonant charger can be configured with threelevels of regulation control. Somewhat crude regulation is provided bythe energy calculator and the opening of switch Q1 during the chargingcycle. As the voltage on the CO capacitor banks nears the target value,the de-qing switch is closed, stopping the resonant charging when thevoltage on C_(o) is at or slightly above the target value. In apreferred embodiment, the switch Q1 and the de-qing switch is used toprovide regulation with accuracy better than +/−0.1%. If additionalregulation is required, the third control over the voltage regulationcould be utilized. This is the bleed-down circuit of switch Q3 and R3(shown at 216 in FIG. 5B) to discharge the CO's down to the precisetarget value.

Improvements Downstream of the CO's

[0053] As indicated above, the pulse power system of the MO and the PAof the present invention each utilizes the same basic pulse power design(FIG. 3A) as was used in the prior art systems. However, changes wererequired for the approximate factor of 3 increase in heat load resultingfrom the greatly increased repetition rate.

Detailed Commutator and Compression Head Description

[0054] In this section, we describe details of fabrication of thecommutator and the compression head.

Solid State Switch

[0055] Solid state switch 46 is an P/N CM 800 HA-34H IGBT switchprovided by Powerex, Inc. with offices in Youngwood, Pa. In a preferredembodiment, two such switches are used in parallel.

Inductors

[0056] Inductors 48, 54 and 64 are saturable inductors similiar to thoseused in prior systems as described in U.S. Pat. Nos. 5,448,580 and5,315,611. side of the C₁ capacitor bank 52.

[0057] one of the induction units of the 1:25 step up pulse transformer56. The housing 545 is connected to the ground lead of unit 56.

Capacitors

[0058] Capacitor banks 42, 52, 62 and 82 (i.e., C_(o), C₁, C_(p-1) andC_(p)) as shown in FIG. 5 are all comprised of banks of off-the-shelfcapacitors connected in parallel. Capacitors 42 and 52 are film typecapacitors available from suppliers such as Vishay Roederstein withoffices in Statesville, N.C. or Wima of Germany. Capacitor bank 62 and64 is typically composed of a parallel array of high voltage ceramiccapacitors from vendors such as Murata or TDK, both of Japan. In apreferred embodiment for use on this ArF laser, capacitor bank 82 (i.e.,C_(p)) comprised of a bank of thirty three 0.3 nF capacitors for acapacitance of 9.9 nF; C_(p-1) is comprised of a bank of twenty four0.40 nF capacitors for a total capacitance of 9.6 nF; C₁ is a 5.7 μFcapacitor bank and C_(o) is a 5.3° F. capacitor bank.

Pulse Transformer

[0059] Pulse transformer 56 is also similar to the pulse transformerdescribed in U.S. Pat. Nos. 5,448,580 and 5,313,481; however, the pulsetransformers of the present embodiment has only a single turn in thesecondary winding and 24 induction units equivalent to 1/24 of a singleprimary turn for an equivalent step-up ratio of 1:24. The secondary ofthe transformer is a single OD stainless steel rod mounted within atight fitting insulating tube of PTFE (Teflon®). The transformerprovides a step-up ratio of 1:25. Thus, an approximately −1400 voltinput pulse will produce an approximately −35,000 volt pulse on thesecondary side. This single turn secondary winding design provides verylow leakage inductance permitting extremely fast output rise time.

Details of Laser Chamber Electrical Components

[0060] The Cp capacitor 82 is comprised of a bank of thirty-three 0.3 nfcapacitors mounted on top of the chamber pressure vessel. (Typically anArF laser is operated with a lasing gas made up of 3.5% argon, 0.1%fluorine, and the remainder neon.) The electrodes are about 28 incheslong which are separated by about 0.5 to 1.0 inch preferably about ⅝inch. Preferred electrodes are described below. In this embodiment, thetop electrode is referred to as the cathode and the bottom electrode isconnected to ground as indicated in FIG. 5 and is referred to as theanode.

Discharge Timing

[0061] In ArF, KrF and F₂ electric discharge lasers, the electricdischarge lasts only about 50 ns (i.e., 50 billionths of a second). Thisdischarge creates a population inversion necessary for lasing action butthe inversion only exists during the time of the discharge. Therefore,an important requirement for an injection seeded ArF, KrF or F₂ laser isto assure that the seed beam from the master oscillator passes throughdischarge region of the power amplifier during the approximately 50billionth of a second when the population is inverted in the laser gasso that amplification of the seed beam can occur. An important obstacleto precise timing of the discharge is the fact that there is a delay ofabout 5 microseconds between the time switch 42 (as shown in FIG. 5) istriggered to close and the beginning of the discharge which lasts onlyabout 40-50 ns. It takes this approximately 5 microseconds time intervalfor the pulse to ring through the circuit between the C₀'s and theelectrodes. This time interval varies substantially with the magnitudeof the charging voltage and with the temperature of the inductors in thecircuit.

[0062] Nevertheless in the preferred embodiment of the present inventiondescribed herein, Applicants have developed electrical pulse powercircuits that provide timing control of the discharges of the twodischarge chambers within a relative accuracy of less than about 2 ns(i.e., 2 billionths of a second). A block diagram of the two circuitsare shown in FIG. 4.

[0063] Applicants have conducted tests which show that timing varieswith charging voltage by approximately 5-10 ns/volt. This places astringent requirement on the accuracy and repeatability of the highvoltage power supply charging the charging capacitors. For example, iftiming control of 5 ns is desired, with a shift sensitivity of 10 ns pervolt, then the resolution accuracy would be 0.5 Volts. For a nominalcharging voltage of 1000 V, this would require a charging accuracy of0.05% which is very difficult to achieve especially when the capacitorsmust be charged to those specific values 4000 times per second.

[0064] Applicants' preferred solution to this problem is to charge thecharging capacitor of both the MO and the PA in parallel from the singleresonant charger 7 as indicated in FIG. 1 and FIG. 4 and as describedabove. It is also important to design the two pulsecompression/amplification circuits for the two systems so that timedelay versus charging voltage curves match as shown in FIG. 4A. This isdone most easily by using to the extent possible the same components ineach circuit.

[0065] Thus, in order to minimize timing variations (the variations arereferred to as jitter) in this preferred embodiment, Applicants havedesigned pulse power components for both discharge chambers with similarcomponents and have confirmed that the time delay versus voltage curvesdo in fact track each other as indicated in FIG. 4A. Applicants haveconfirmed that over the normal operating range of charging voltage,there is a substantial change in time delay with voltage but the changewith voltage is virtually the same for both circuits. Thus, with bothcharging capacitors charged in parallel charging voltages can be variedover a wide operating range without changing the relative timing of thedischarges.

[0066] Temperature control of electrical components in the pulse powercircuit is also important since temperature variations can affect pulsecompression timing (especially temperature changes in the saturableinductors). Therefore, a design goal is to minimize temperaturevariations and a second approach is to monitor temperature of thetemperature sensitive components and using a feedback control adjust thetrigger timing to compensate. Controls can be provided with a processorprogrammed with a learning algorithm to make adjustments based onhistorical data relating to past timing variations with known operatinghistories. This historical data is then applied to anticipate timingchanges based on the current operation of the laser system.

Trigger Control

[0067] The triggering of the discharge for each of the two chambers isaccomplished separately utilizing for each circuit a trigger circuitsuch as one of those described in U.S. Pat. No. 6,016,325. Thesecircuits add timing delays to correct for variations in charging voltageand temperature changes in the electrical components of the pulse powerso that the time between trigger and discharge is held as constant asfeasible. As indicated above, since the two circuits are basically thesame, the variations after correction are almost equal (i.e., withinabout 2 ns of each other).

Techniques to Control Discharge Timing

[0068] Since the relative timing of the discharges can have importanteffects on beam quality additional steps may be justified to control thedischarge timing. For example, some modes of laser operation may resultin wide swings in charging voltage or wide swings in inductortemperature. These wide swings could complicate discharge timingcontrol.

[0069] Monitor Timing

[0070] The timing of the discharges can be monitored on a pulse-to-pulsebasis and the time difference can be used in a feedback control systemto adjust timing of the trigger signals closing switch 42. Preferably,the PA discharge would be monitored using a photocell to observedischarge fluorescence (called ASE) rather than the laser pulse sincevery poor timing could result if no laser beam being produced in the PA.For the MO either the ASE or the seed laser pulse could be used.

[0071] Bias Voltage Adjustment

[0072] The pulse timing can be increased or decreased by adjusting thebias currents through inductors L_(B1) L_(B2) and L_(B3) which providebias for inductors 48, 54 and 64 as shown in FIG. 3A. Other techniquescould be used to increase the time needed to saturate these inductors.For example, the core material can be mechanically separated with a veryfast responding PZT element which can be feedback controlled based on afeedback signal from a pulse timing monitor.

[0073] Adjustable Parasitic Load

[0074] An adjustable parasitic load could be added to either or both ofthe pulse power circuits downstream of the CO's.

[0075] Additional Feedback Control

[0076] Charging voltage and inductor temperature signals, in addition tothe pulse timing monitor signals can be used in feedback controls toadjust the bias voltage or core mechanical separation as indicated abovein addition to the adjustment of the trigger timing as described above.

Alternate Pulse Power Circuit

[0077] A second preferred pulse power circuit is shown in FIGS. 5C1, 5C2and 5C3. This circuit is similar to the one described above but utilizesa higher voltage power supply for charging C₀ to a higher value. As inthe above described embodiments, a high voltage pulse power supply unitoperating from factory power at 230 or 460 volts AC, is power source fora fast charging resonant charger as described above and designed forprecise charging two 2.17 μF at frequencies of 4000 to 6000 Hz tovoltages in the range of about 1100 V to 2250 V. The electricalcomponents in the commutator and compression head for the masteroscillator are as identical as feasible to the corresponding componentsin the power amplifier. This is done to keep time responses in the twocircuits as identical as feasible. Switches 46 are banks of two IGBTswitches each rated at 3300 V and arranged in parallel. The C₀ capacitorbanks 42 is comprised of 1280.068 μF 1600 V capacitors arranged in 64parallel legs to provide the 2.17 μF C₀ bank. The C₁ capacitor banks 52are comprised of 1360.068 μF 1600 V capacitors arranged in 68 parallellegs to provide a bank capacitance of 2.33 μF. The C_(p-1) and C_(p)capacitor banks are the same as those described above with reference toFIG. 5. The 54 saturable inductors are single turn inductors providingsaturated inductance of about 3.3 nH with five cores comprised of 0.5inch thick 50%-50% Ni—Fe with 4.9 inch OD and 3.8 inch ID. The 64saturable inductors are two turn inductors providing saturatedinductance of about 38 nH each comprised of 5 cores, 0.5 inch thick madewith 80%-20% Ni-Fe with an OD of 5 inches and an ID of 2.28 inches.Trigger circuits are provided for closing IGBT's 46 with a timingaccuracy of two nanoseconds. The master oscillator is typicallytriggered about 40 ns prior to the triggering of the IGBT 46 for poweramplifier. However, the precise timing is preferably determined byfeedback signals from sensors which measure the timing of the output ofthe master oscillator and the power amplifier discharge.

Alternate Technique for Timing Control

[0078] As described earlier, the throughput timing of the magnetic pulsecompression in the Pulsed Power system is dependent upon the magneticmaterial properties that can be a function of the material temperature,etc. In order to maintain precise timing, it is therefore extremelyimportant to either directly or indirectly monitor and/or predict thesematerial properties. One method described previously would utilizetemperature monitors along with previously collected data (delay time asa function of temperature) to predict the timing.

[0079] An alternate approach would utilize the magnetic switch biascircuit to actually measure the magnetic properties (the saturationtime) as the magnetics were reverse biased in between pulses (or priorto the first pulse). The bias circuit would apply sufficient voltage tothe magnetic switch to reverse bias the material and at the same timemeasure the saturation time so that the laser timing could be accuratelycontrolled. Since the volt-second product utilized in reverse biasingthe switch should be equal to that required during normal dischargeoperation in the forward direction, the throughput delay time of thePulsed Power system could be easily calculated knowing the operatingvoltage of the upcoming pulse.

[0080] A schematic diagram of the proposed approach is shown in FIG. 5D.Initial operation assumes that the magnetic switch, L1, is alreadysaturated in the forward direction, provided by power supply BT1 throughthe two bias isolation inductors, Lbias, and switch S4. This current isthen interrupted by opening S4 and closing S2 which applies ˜100V to themagnetic switch, L1, which then saturates after ˜30 us. A timer istriggered when S2 closes and stops counting when a current probe detectssaturation of L1, thus calculating the saturation time of L1 for the100V applied voltage. L1 is now reverse biased and ready for the mainpulse discharge sequence once residual voltage has been drained from thecircuit by S3 and other components.

Burst Type Operation

[0081] Feedback control of the timing is relatively easy and effectivewhen the laser is operating on a continuous basis. However, normallylithography lasers operate in a burst mode such as the following toprocess 20 areas on each of many wafers:

[0082] Off for 1.2 minutes to move a wafer into place

[0083] 4000 Hz for 0.2 seconds to illuminate area 1

[0084] Off for 0.3 seconds to move to area 2

[0085] 4000 Hz for 0.2 seconds to illuminate area 2

[0086] Off for 0.3 seconds to move to area 3

[0087] 4000 Hz for 0.2 seconds to illuminate area 3

[0088] 4000 Hz for 0.2 seconds to illuminate area 199

[0089] Off for 0.3 seconds to move to area 200

[0090] 4000 Hz for 0.2 seconds to illuminate area 200

[0091] Off for one minute to change wafers

[0092] 4000 Hz for 0.2 seconds to illuminate area 1 on the next wafer,etc.

[0093] This process may be repeated for many hours, but will beinterrupted from time-to-time for periods longer or shorter than 1.2minute.

[0094] The length of down times will affect the relative timing betweenthe pulse power systems of the MO and the PA and adjustment may berequired in the trigger control to assure that the discharge in the PAoccurs when the seed beam from the MO is at the desired location. Bymonitoring the discharges and the timing of light out from each chamberthe laser operator can adjust the trigger timing (accurate to withinabout 2 ns) to achieve best performance.

[0095] Preferably a laser control processor is programmed to monitor thetiming and beam quality and adjust the timing automatically for bestperformance. Timing algorithms which develop sets of bin valuesapplicable to various sets of operating modes are utilized in preferredembodiments of this invention. These algorithms are in preferredembodiments designed to switch to a feedback control during continuousoperation where the timing values for the current pulse is set based onfeedback data collected for one or more preceding pulse (such as theimmediately preceding pulse).

No Output Discharge

[0096] Timing algorithms such as those discussed above work very wellfor continuous or regularly repeated operation. However, the accuracy ofthe timing may not be good in unusual situations such as the first pulseafter the laser is off for wafer change or for longer periods such as 5minutes. In some situations imprecise timing for the first one or twopulses of a burst may not pose a problem. A preferred technique is topreprogram the laser so that the discharges of the MO and the PA areintentionally out of sequence for one or two pulses so thatamplification of the seed beam from the MO is impossible. For example,laser could be programmed to trigger the discharge of the PA 110 nsprior to the trigger of the MO. In this case, there will be nosignificant output from the laser but the laser metrology sensors candetermine the timing parameters so that the timing parameters for thefirst output pulse is precise.

Applicants Test

[0097] Applicants have conducted careful experiments to measure theimpact of the relative timing of the discharge of the master oscillatorand the power amplifier. These tests are summarized in FIG. 5 in whichthe Applicants have plotted the pulse energy (in millijoules) ofamplified stimulated emission (ASE) from the output of the poweramplifier and the line narrowed output (also in milliJoules). Both plotsare made as a function of delay between the beginning of discharge ofthe master oscillator and the beginning of discharge of the poweramplifier. The reader should note that the energy scale of the ASE issmaller than that for the line narrowed light output.

[0098] Lithography customer specifications call for the ASE to be a verysmall fraction of the line narrowed laser output. A typicalspecification calls for the ASE to be less than 5×10⁻⁴ times the linenarrowed energy for a thirty pulse window. As is shown in FIG. 5 the ASEis substantially zero when the narrow band pulse is maximum; i.e., inthis case when the MO discharge precedes the PA discharge by between 25and 40 ns. Otherwise, the ASE becomes significant.

[0099] As described above, the MO and the PA pulse power circuits can betriggered with a timing accuracy of about 2 ns so with good feedbackinformation regarding the response of the two pulse power circuits, theMO and the PA can be discharged within the range where line narrowedpulse energy is maximum and ASE is insignificant. Therefore, forcontinuous operation with good feedback control, control of the twosystems is relatively easy. However, typical operation of these lasersis burst mode operation as described above. Therefore, the first pulseof a burst is likely to produce bad results because any feedback datawill be out of date and temperature changes in the electrical componentswill likely affect their responses.

[0100] One solution is to initiate a test pulse prior to each burst(perhas with the laser shutter closed) so that up-to-date timing datacan be obtained. This solution is not desirable for several reasonsincluding the delay associated with closing and opening the shutter.

[0101] A better solution is the one referred to briefly above in whichthe two chambers are caused to discharge at relative times chosen sothat there can be no amplification of the output of the MO. This means,in the case of the system that is the subject of the FIG. 5 data, thatthe MO must be discharged later than about 40 ns after the PA isdischarged or that the MO must be discharged earlier than 110 ns priorto the discharge of the PA. FIGS. 6 and 7 describe these two jittercontrol techniques.

[0102] In the FIG. 6 technique if more than one minute has elapsed sincethe previous pulse, the PA is discharged 110 ns after the MO isdischarged. Otherwise the PA is discharged 30 ns after the MO isdischarged to produce the desired pulse energy. The technique calls forcollecting timing data, and feedback corrections are made for anychanges in timing between trigger and discharge. The discharge aredetected by photocells detecting discharge produced ASE light in boththe MO and the PA.

[0103] In the FIG. 7 technique if more than one minute has elapsed sincethe previous pulse, the MO is discharged 40 ns after to the discharge ofthe PA. As before, timing data is collected and used to assure thatdischarges subsequent to the first pulse occur when they should toproduce maximum narrow band output and minimum ASE.

[0104] Thus, the first pules of each burst after more than a one minuteidle time produces substantially zero line narrowed output on anextremely small amount of ASE. Applicants estimate that the ASE forpulse window of at least 30 pulse the ASE will be less than 2×10⁻⁴ ofthe integrated narrow band energy. Since pulses in this preferred laserare at the rate of 4000 pulses per second, the loss of a single pulse atthe beginning of a burst of pulses is not expected to be a problem forthe laser users.

Variations

[0105] Many modifications could be made to the procedures outline inFIGS. 6 and 7 to achieve similar results. The time values such as the 30second targets shown of course should be chosen to provide best results.The 1 minute could be as small as a few milliseconds so that the firstpulse of each burst is thrown away. In the FIG. 6 situations based onthe FIG. 5 data, the 110 ns time period could be shortened to as much asabout 70 ns and in the FIG. 7 situation the 40 ns time period could beas short as about 20 ns. The programs could be modified to provide fortwo or several no output discharges at the start of each burst or at thestart of each burst following an extended idle period. Parameters otherthan the P-cell outputs threshold could be used to determine the timesof beginning of discharge. For example, the peaking capacitor voltagecould be monitored. The sudden drop in voltage soon after the beginningof discharge could be used as the time of start of discharge.

[0106] While the present invention has been described in terms ofspecific embodiments persons skilled in the art will recognize manymodifications could be made within the general scope of the invention.For example, additional data could be collected to provide additionalfeedback information to possibly improve timing precision. It is knownthat temperature of the electrical components affect timing so thetemperature of the components could be monitored and data collectedcould be correlated with historical timing data collected as a functionof temperature and appropriate corrections could be included in thealgorithms shown in FIGS. 6 and 7. Other techniques could be used todetermine the timing responses of the pulse power components. Forexample, the saturable reactors in the pulse power circuits produce muchof the timing variations. A test voltage could be applied across therereactors to determine their response and data collected could be used tocorrect discharge timing. Accordingly, the above disclosure is notintended to be limiting and the scope of the claims should be determinedby the appended claims and their legal equivalents.

We claim:
 1. A very narrow band two chamber high repetition rate gasdischarge laser system with special timing control features, said systemcomprising: A) a first laser unit comprising: 1) a first dischargechamber containing; a) a first laser gas b) a first pair of elongatedspaced apart electrodes defining a first discharge region, 2) a firstfan for producing sufficient gas velocities of said first laser gas insaid first discharge region to clear from said first discharge region,following each pulse, substantially all discharge produced ions prior toa next pulse when operating at a repetition rate in the range of 4,000pulses per second or greater, 3) a first heat exchanger system capableof removing at least 16 kw of heat energy from said first laser gas, 4)a line narrowing unit for narrowing spectral bandwidths of light pulsesproduced in said first discharge chamber, B) a second laser unitcomprising: 1) a second discharge chamber containing: a) a second lasergas, b) a second pair of elongated spaced apart electrodes defining asecond discharge region 2) a second fan for producing sufficient gasvelocities of said second laser gas in said second discharge region toclear from said second discharge region, following each pulse,substantially all discharge produced ions prior to a next pulse whenoperating at a repetition rate in the range of 4,000 pulses per secondor greater, 3) a second heat exchanger system capable of removing atleast 16 kw of heat energy from said second laser gas, C) a pulse powersystem configured to provide electrical pulses to said first pair ofelectrodes and to said second pair of electrodes sufficient to producelaser pulses at rates of about 4,000 pulses per second with preciselycontrolled pulse energies in excess of about 5 mJ, D) a laser beammeasurement and control system for measuring pulse energy, wavelengthand bandwidth of laser output pulses produced by said two chamber lasersystem and controlling said laser output pulses in a feedback controlarrangement, and E) a processor programmed with an algorithm providingfeedback timing control.
 2. A laser system as in claim 1 wherein saidfirst laser unit is configured as a master oscillator and said secondlaser unit is configured as a power amplifier.
 3. A laser system as inclaim 2 wherein said laser gas comprises argon, fluoride and neon.
 4. Alaser system as in claim 2 wherein said laser gas comprises krypton,fluorine and neon.
 5. A laser system as in claim 2 wherein said lasergas comprises fluorine and a buffer gas chosen from a group consistingof neon, helium or a mixture of neon and helium.
 6. A laser as in claim1 wherein said first fan and said second fan each are tangential fansand each comprises a shaft driven by two brushless DC motors.
 7. A laseras in claim 15 wherein said motors are water cooled motors.
 8. A laseras in claim 1 wherein said pulse power power system comprise watercooled electrical components.
 9. A laser as in claim 8 wherein at leastone of said water cooled components is a component operated at highvoltages in excess of 12,000 volts.
 10. A laser as in claim 9 whereinsaid high voltage is isolated from ground using an inductor throughwhich cooling water flows.
 11. A laser as in claim 1 wherein said pulsepower system comprises a resonant charging system to charge a chargingcapacitor to a precisely controlled voltage.
 12. A laser system as inclaim 1 wherein said system is configured to operate either of a KrFlaser system, an ArF laser system or an F₂ laser system with minormodifications.
 13. A laser system as in claim 1 wherein substantiallyall components are contained in a laser enclosure but said systemcomprises an AC/DC module physically separate from the enclosure.
 14. Alaser system as in claim 1 wherein said pulse power system comprises amaster oscillator charging capacitor bank and a power amplifier chargingcapacitor bank and a resonant charger configured to charge both chargingcapacitor banks in parallel.
 15. A laser as in claim 14 wherein saidpulse power system comprises a power supply configured to furnish atleast 2000V supply to said resonant charges.
 16. A laser as in claim 1and further comprising a gas control system for controlling F₂concentrations in said first laser gas to control master oscillator beamparameters.
 17. A laser as in claim 1 and further comprising a gascontrol system for controlling laser gas pressure in said first lasergas to control master oscillator beam parameters.
 18. A laser as inclaim 2 and further comprising a discharge timing controller fortriggering discharges in said power amplifier to occur between 20 and 60ns after discharges in said master oscillator.
 19. A laser as in claim 2and further comprising a discharge controller programmed to cause insome circumstances discharges so timed to avoid any significant outputpulse energy.
 20. A laser as in claim 19 wherein said controller isprogrammed to cause discharge, in some circumstances, in said poweramplifier at least 20 ns prior to discharge in said master oscillator.21. A laser as in claim 19 wherein said controller is programmed tocause discharge, in some circumstances, in said power amplifier at least70 ns after discharge in said master oscillator discharge.
 22. A laseras in claim 20 wherein said at least 20 ns is about 40 ns.
 23. A laseras in claim 21 wherein said at least 70 ns is about 110 ns.
 24. A laseras in claim 1 wherein said laser also comprises P-cells for measuringASE of discharge in each of said first and second chamber.
 25. A laseras in claim 24 wherein signals from said P-cells wherein said controlleris programmed to use signals from said P-cells to indicate discharges.26. A laser as in claim 21 wherein said controller is programmed todetermine discharges based on measurement of capacitor voltage.
 27. Aprocess for controlling discharge timing of a burst of pulses producedby a MOPA laser system comprising the steps determining the timing ofdischarges to produce said pulses based on feedback discharge timingsignal wherein at least a first set of discharges at start of said burstof pulses are programmed to occur at relative times so that nosignificant lasing results as a consequence of the discharge.